1. Field of the Invention
The present invention relates to a so-called double-surface-packaged semiconductor device in which IC packages are mounted on two opposed surfaces of a circuit board.
2. Description of the Related Art
FIG. 14 is a cross-sectional view of a conventional semiconductor device of double-surface-mounted type. In FIG. 14, reference numeral 1 denotes a circuit board, and reference numerals 2 and 3 are IC packages. Each of the IC packages 2 and 3 is a surface-mounted type package which includes a rectangular package body 2a or 3a, and gull-wing shaped lead terminals 2b or 3b extending from opposite sides of the package body 2a or 3a. The circuit board 1 has mount pads 4 formed on each of the upper and lower surfaces thereof. The mount pads 4 are disposed on the upper and lower main surfaces at positions which are symmetrical with respect to the circuit board 1. The lead terminals 2b and 3b of the IC packages 2 and 3 are fixed to the mount pads 4 by solder 5 and 6, respectively.
When the IC packages 2 and 3 are to be mounted on both surfaces of the circuit board 1, the solder 5 is first placed on the mount pads 4 formed on the upper surface of the circuit board 1. Next, the lead terminals 2b of the IC package 2 are placed on the solder 5, and the solder 5 is heated and thereby melted, by which the mount pads 4 are electrically connected to the lead terminals 2b. Thereafter, the IC package 3 is mounted on the lower surface of the circuit board 1 in the same manner.
In the aforementioned conventional semiconductor device, since the IC packages 2 and 3 are mounted on upper and lower surfaces of the circuit board 1 at positions which are symmetrical with respect to the circuit board 1, the conventional semiconductor device has the following drawbacks.
When the IC packages 2 and 3 are mounted on the circuit board 1 or in a temperature cycle test which is performed after fabrication, since the coefficient of thermal expansion of the IC packages 2 and 3 is different from that of the circuit board 1 and the amount of deformation due no changes in the temperature thus differs between the IC packages 2 and 3 and the circuit board 1, a stress, such as tension or compression, is exerted on the solder 5 and 6 connecting the IC packages 2 and 3 and the circuit board 1. Consequently, cracks may occur in the solder 5 and 6, degrading the electrical characteristics of the semiconductor device.